A wavelength division multiplexing (WDM) system comprises a plurality of transmitters and receivers for transmitting data at different wavelengths or channels through the system, and generally a plurality of in-line amplifiers positioned at spaced apart intervals along the system for boosting the power of light being transmitted through the system. The amplifiers are typically rare earth doped amplifiers, and a part of the system between a transmitter and receiver is known as a link.
Generally, such WDM systems are installed by equipment vendors, and operators offering telecommunications services to customers who buy the systems from the equipment vendors.
To allow for ageing in the in line amplifiers and other components and to compensate for losses incurred in cable repairs, the operators build in significant levels of overall optical signal to noise ratio (OSNR) margin into the system at the outset. This in effect reduces the capacity of the link by as much as four times.
In addition, it is well know that when data is transmitted via a transmission system, errors may be introduced into the data due to noise in the system.
The longer the distance over which the data is transmitted, the more errors are likely to be introduced to the data.
Operators generally carry a mixture of classes of traffic through WDM line systems. These different classes of traffic are related to different service level agreements (SLAs). The highest revenue earning traffic will be referred to herein as A class traffic and comprises for example data from banks and other institutions which must be able to guarantee complete security and integrity of the data transmitted through the system. Such traffic is often provided on a fully protected basis in order to minimise outages that may occur when the system needs repairing.
The lowest class will be referred to herein as B class traffic and may comprise for example data from private individuals requiring access to the Internet. The B class traffic earns the least revenue and is often provided on a best efforts basis. In general, the volume of A class traffic will be significantly lower than lower tariff (B class) traffic.
It is known that rare earth doped amplifiers have different power capacities at different wavelengths. However, in recent years with the emergence of filtering techniques, such as Bragg Gratings, it is possible flatten the power capacity curve of rare earth doped amplifiers so that the gain for each separate wavelength is more or less equal. Despite this flattening, long systems still exhibit a notable lack of gain flatness and it is therefore necessary to pre-emphasise certain channels to ensure that each channel is nominally equivalent to each other channel. Pre-emphasis is achieved by putting more power into the poorer channels than the better channels. This means that at the receiver all channels appear equal.
In older transmission systems, where the level of gain flattening sophistication is lower, more attention to pre-emphasis is required to overcome significant levels of non-flatness.
When setting up a WDM link, it is necessary to calculate a power budget for the link. This is generally a co-operative process taking place between the equipment vendor and the operator. When working out the power budget, an ageing margin of several dBs will be set aside to compensate for cable repairs and system/component ageing. Damage to the system can be either permanent and caused by repairs or permanent deterioration in either the transmission medium or the in-line optical elements such as the transmitter receivers and amplifiers. Alternatively, damage may be transient. Polarisation Mode Dispersion (PMD) is a significant transient phenomena, being particularly deleterious at high transmission rates such as 10 Gbit/s and 40 Gbit/s.
The ageing margin is designed into the system at the outset. This exercise normally entails the generation of a power budget for a particular link, making an estimate of how the various elements will deteriorate over the lifetime of the system and building this into the link power budget. Due to the fact that transmission technologies are continually evolving it is difficult to make an estimate of these ageing elements. It is therefore common to take a very conservative position and it is not uncommon to build in an ageing margin of typically >5 dB on the most important links in the system such as long haul submarine links. This ageing margin can therefore be considered as 5 dB or more worth of lost power capacity throughout the lifetime of the system.